The well being of a mammal (e.g. human) may be assessed and monitored by determining the level of oxygen saturation in the blood or tissue using the method of oximetry or pulse oximetry. Conventional pulse oximetry uses an apparatus that emits light of at least two wavelengths to irradiate a tissue volume and collects light that propagated through a portion of the tissue containing pulsating blood. The collected light at the two wavelengths is analyzed to determine the heart rate of the subject and the degree of oxygen saturation of the tissue or blood being monitored. Oximetry and pulse oximetry can be performed in transmission or reflection geometries. However, it can provide inaccurate readings in the presence of ambient light, in cases of low perfusion of blood and during motion of the subject or the portion of tissue being examined.
The well-being of the fetus inside the uterus is traditionally monitored by measuring the fetal-heart-rate (FHR) by placing sensors on the skin of the mother's abdomen directed at the fetus or the fetal heart. A normal fetal-heart-rate (FHR) pattern is usually associated with the delivery of a normal well-oxygenated infant; however, a non-reassuring FHR is not always associated with the delivery of a compromised infant.
In the case of a non-reassuring FHR, the fetal blood oxygen saturation level can be measured indirectly by either fetal head sampling, which measures the pH level of the fetal blood, or by directly attaching a pulse oximeter to the presenting part of the fetal head during labor. Both of these methods are performed following the rupture of membranes where the fetal head and/or cheeks can be reached.
Assessing the maturity of fetal lungs is one of the major concerns of pre-term deliveries. If the baby is delivered and the lungs are not mature, the baby may develop Respiratory Distress Syndrome (RDS), which can result either in fetal death or in long-lasting periods of repeated respiratory difficulty.
In cases where intervention is considered in the course of pregnancy (such as caesarean section or induction of labor) and there is a need to assess the maturity of the lungs, amniotic fluid is drained. Measuring phospholipids in amniotic fluid as the lecithin/sphingomyelin ratio using thin-layer chromatography has been the established clinical procedure for predicting fetal lung maturity. Although it is the clinical gold standard method, it remains a time-consuming process, has a large intralaboratory and interlaboratory coefficient of variation, and requires expertise. In addition, the procedure of amniotic fluid drainage itself is invasive and suffers a small risk of abortion. Additional techniques that are used for assessing lung maturity levels include measuring the number of lamellar bodies in a volume of amniotic fluid, measuring the prostaglandin level in amniotic fluid and measuring the fluorescence polarization of a sampled amniotic fluid.
When a fetus is acutely distressed, for example as a result of strangulation by the umbilical cord, the bowel content, meconium, may be passed into the amniotic fluid. Assessment of meconial contamination of amniotic fluid is important in the management of late pregnancy. It appears in nearly one third of all fetuses by 42 weeks of gestation. In cases where the fetus gasps during delivery, inhaling the sticky meconium into the upper respiratory tract results in partial airways obstruction. Meconium aspiration syndrome occurs in 0.2% to 1% of all deliveries and has a mortality rate as high as 18%. The disease is responsible for 2% of all prenatal deaths.
To date, meconium stained amniotic fluid is diagnosed following the rupture of membranes, when the amniotic fluid is drained. However, in cases where the fetus head is tightly fitted in the pelvis, the amniotic fluid is not drained, preventing detection of any potentially harmful outcome to the respiratory tract.
Several methods for determining the optical properties of a tissue in a body are known in the art. These include: near infrared spectroscopy (for example using time resolved spectroscopy as disclosed in U.S. Pat. No. 5,555,885), ultrasound tagging of light (for example Yao G et al. Applied Optics Vol. 39 pg 659-664, February 2000) and photoacoustic spectroscopy (for example Oraevsky A. A. et al. Applied Optics Vol. 36 pg 402-405, January 1997). In each of these technologies electromagnetic radiation is used to interact with the tissue, where the interaction depends on the absorption and scattering properties of the components composing the tissue or fluid. Consequently by monitoring this interaction at one or several wavelengths one can extract the optical properties of the tissue and determine the concentration of its components. For imaging a tissue in two or three dimensions the signals are collected and analyzed per pixel or voxel in the target volume. In order to determine the local concentrations of different components tomographic algorithms are used when the light distribution is detected (as in near infrared spectroscopy and ultrasound tagged light) or time dependent signals are analyzed in cases where acoustic energy is detected (as in photoacoustic spectroscopy). The major research and development effort in the above mentioned technologies has been to improve imaging resolution and sensitivity per pixel or voxel.
U.S. Pat. No. 6,498,942 discloses an apparatus for optoacoustic monitoring of blood oxygenation. The apparatus includes a radiation source of pulsed radiation and a probe having a front face to be placed in close proximity to or in contact with a tissue site of an animal body. The probe further includes a plurality of optical fibers terminating at the surface of the front face of the probe and connected at their other end to a pulsed laser. The front face of the probe also has mounted therein or thereon a transducer for detecting an acoustic response from blood in the tissue site to the radiation pulses connected to a processing unit which converts the transducer signal into a measure of venous blood oxygenation